Combustion, Fuels, Burners, and Draft
What this domain tests
This domain tests combustion chemistry, fuel properties, burner operation, and draft. Expect questions on excess air, O2 and CO2 readings, draft measurement, fuel oil grades, gas train components, and flame safeguards.
Combustion basics
- Stoichiometric air is the exact air needed for complete combustion; practical firing uses excess air.
- O2 in flue gas indicates excess air. CO indicates incomplete combustion and must be corrected.
- High stack temperature signals heat loss; very low O2 can produce CO and smoke.
- Draft types include natural, forced, induced, and balanced. The furnace should normally be slightly negative, often about -0.05 to -0.1 in. w.c.
Fuels and burners
- Natural gas is lighter than air and has a high heating value; the gas train includes a regulator, shutoff valves, and leak-test provisions.
- No. 2 fuel oil is a light distillate; No. 6 is heavy and usually requires heating and good atomization.
- Common burner types include atmospheric, power, and rotary-cup burners. Flame safeguards prove flame and shut off fuel on failure.
Worked example
A flue gas analyzer reads 4% O2. Using the dry-flue approximation, excess air = O2 / (20.9 - O2) × 100 = 4 / 16.9 × 100 = about 23.7%. If CO appears, do not chase a lower O2 reading; correct the burner or air/fuel ratio first. If stack temperature rises at the same firing rate, check for fouling or draft problems.
Traps
- Confusing CO2 with CO; CO is the dangerous incomplete-combustion product.
- Assuming more excess air is always better; it lowers efficiency.
- Measuring draft with a gauge that is not zeroed or using the wrong scale.
- Using a combustible gas meter as an oxygen analyzer.